Power dividers have been used for many years to divide a single input signal into two or more output signals. These output signals may or may not have the same power level. Historically, in microwave frequency applications, these dividers have been constructed of transmission lines of appropriate impedances. Examples are illustrated by Cohn in "A Class of Broadband Three-Port TEM-Mode Hybrids", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-16, No. 2, February 1968, pages 110-116; by Saleh in "Planar Electrically Symmetric n-Way Hybrid Power Dividers/Combiners", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-28, No. 6, June 1980, pages 555-563; by Wahi in "Wideband, Unequal Split Ratio Wilkinson Power Divider", Microwave Journal, September 1985, pages 205-209; by Hanna et al. in "A Wide-Band 12-GHz 12-Way Planar Power Divider/Combiner", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-34, No. 8, August 1986, pages 896-897; by Madihian et al. in "GaAs-Monolithic IC's for an X-Band PLL-Stabilized Local Source", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-34, No. 6, June 1986, pages 707-713; by Bosisio et al. in "Computer-Aided Evaluation of Manufacturing Tolerances of an Optimized, Wideband Wilkinson Power Divider", Microwave Journal, April 1987, pages 155-166; by Wang et al. in "Manufacturing Pass Rate Predictions of an Optimized 2 to 18 GHz Wilkinson Power Divider in Microstripline", Microwave Journal, December 1987, pages 115-122; and by Hamadallah in "Microstrip Power Dividers at mm-Wave Frequencies", Microwave Journal, July 1988, pages 115-127. These devices tend to provide high isolation, such as more than 20 dB, only over very narrow frequency ranges.
Isolation over somewhat broader bandwidths has been achieved while using less circuit area by forming the power dividers out of lumped elements. Two examples are Burgess, "Unequal Power Division with a Lumped Element Divider", RF Design, June 1988, pages 69-73; and Beckwith et al., "Wide Band Monolithic Power Dividers", Microwave Journal, February 1989, pages 155-160.
Active elements have also been used for frequency and power dividers. These are illustrated by Kanazawa et al. in "A 15 GHz Single Stage GaAs Dual-Gate FET Monolithic Analog Frequency Divider with Reduced Input Threshold Power", IEEE 1988 Microwave and Millimeter-Wave Monolithic Circuits Symposium, pages 47-49; by Barta et al. in "Surface-Mounted GaAs Active Splitter and Attenuator MMIC's Used in a 1-10 GHz Leveling Loop", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-34, No. 12, December 1986, pages 1569-1575, and in "A 2 to 8 GHz Leveling Loop Using a GaAs MMIC Active Splitter and Attenuator", IEEE, 1986, pages 75-79; and by Harvey et al. in "A Low Noise GaAs MMIC Satellite Downconverter for the 6 to 4 GHz Band", 1987 IEEE MTT-S Digest, pages 233-236.
Harvey et al. use a common-gate input stage and a common source output stage. Barta et al. use four common-source FET stages to split the input power between two output power sources to obtain over 24 dB isolation up to 9 GHz.
It is thus desirable to have a power splitter usable for MMIC applications that provides high isolation over a broad bandwidth. This is provided by an active power splitter made according to the present invention.
Generally speaking, this is provided by an active power splitter for splitting power input on an input port between first and second output ports. A pair of transistors are included. Each has a control terminal coupled, such as by electrical connection, to the input port, and two current-conducting terminals. One of the current-conducting terminals of each transistor is coupled to a reference voltage. The other current-conducting terminal of one transistor is coupled to the first output port, and the other current-conducting terminal of the other transistor is coupled to the second output port. A signal input on one of the output ports is out of phase with the same signal passing through the two transistors to the other output port. Impedance is coupled between the two output ports for feeding signals of reduced amplitude between the first and second output ports.
The preferred embodiment of the present invention comprises an input inductor coupled to the input port. A first and a second FET are provided. The gate of each FET is coupled to the input inductor. The source of each FET is coupled to ground potential. The drain of the first FET is coupled to the first output port, and the drain of the second FET is coupled to the second output port. A signal input on the drain of one of the FETs is approximately 180.degree. out of phase with the same signal passing through the two FETs to the drain of the other FET.
First and second impedances couple the gate and drain of the first and second FETs, respectively. A series-connected isolation resistor and inductor are coupled between the first and second output ports for feeding signals of reduced amplitude between the first and second output ports.
Such a circuit provides inherent isolation between the two output ports through the two FETs. Further, a signal passing from one output port through the FETs to the other output port is found to undergo a 180.degree. phase shift. Thus, by inserting a reduced signal without substantial phase change directly between the output ports, the signals become self-cancelling, thereby providing even greater isolation. These results are achieved in a single stage circuit that is readily implemented as an MMIC.
These and other advantages and features of the invention are apparent from the preferred embodiment disclosed in the following detailed description and the associated drawings.